New limit on time-reversal violation in beta decay.

We report the results of an improved determination of the triple correlation DP·(p(e)×p(v)) that can be used to limit possible time-reversal invariance in the beta decay of polarized neutrons and constrain extensions to the standard model. Our result is D=[-0.96±1.89(stat)±1.01(sys)]×10(-4). The corresponding phase between gA and gV is ϕAV=180.013°±0.028° (68% confidence level). This result represents the most sensitive measurement of D in nuclear ß decay.

Measurement of Parity Violation in np Capture: the NPDGamma Experiment.

The NPDGamma experiment will measure the parity-violating directional gamma ray asymmetry A γ in the reaction [Formula: see text]. Ultimately, this will constitute the first measurement in the neutron-proton system that is sensitive enough to challenge modern theories of nuclear parity violation, providing a theoretically clean determination of the weak pion-nucleon coupling. A new beam-line at the Los Alamos Neutron Science Center (LANSCE) delivers pulsed cold neutrons to the apparatus, where they are polarized by transmission through a large volume polarized (3)He spin filter and captured in a liquid para-hydrogen target. The 2.2 MeV gamma rays from the capture reaction are detected in an array of CsI(Tl) scintillators read out by vacuum photodiodes operated in current mode. We will complete commissioning of the apparatus and carry out a first measurement at LANSCE in 2004-05, which would provide a statistics-limited result for A γ accurate to a standard uncertainty of ±5 × 10(-8) level or better, improving on existing measurements in the neutron-proton system by a factor of 4. Plans to move the experiment to a reactor facility, where the greater flux would enable us to make a measurement with a standard uncertainty of ±1 × 10(-8), are actively being pursued for the longer term.

Commissioning of the NPDGamma Detector Array: Counting Statistics in Current Mode Operation and Parity Violation in the Capture of Cold Neutrons on B 4 C and (27) Al.

The NPDGamma γ-ray detector has been built to measure, with high accuracy, the size of the small parity-violating asymmetry in the angular distribution of gamma rays from the capture of polarized cold neutrons by protons. The high cold neutron flux at the Los Alamos Neutron Scattering Center (LANSCE) spallation neutron source and control of systematic errors require the use of current mode detection with vacuum photodiodes and low-noise solid-state preamplifiers. We show that the detector array operates at counting statistics and that the asymmetries due to B4C and (27)Al are zero to with- in 2 × 10(-6) and 7 × 10(-7), respectively. Boron and aluminum are used throughout the experiment. The results presented here are preliminary.

Distribution and dynamics of laser-polarized (129)Xe magnetization in vivo.

The first magnetic resonance imaging studies of laser-polarized (129)Xe, dissolved in the blood and tissue of the lungs and the heart of Sprague-Dawley rats, are described. (129)Xe resonances at 0, 192, 199, and 210 ppm were observed and assigned to xenon in gas, fat, tissue, and blood, respectively. One-dimensional chemical-shift imaging (CSI) reveals xenon magnetization in the brain, kidney, and lungs. Coronal and axial two-dimensional CSI show (129)Xe dissolved in blood and tissue in the thorax. Images of the blood resonance show xenon in the lungs and the heart ventricle. Images of the tissue resonance reveal xenon in lung parenchyma and myocardium. The (129)Xe spectrum from a voxel located in the heart ventricle shows a single blood resonance. Time-resolved spectroscopy shows that the dynamics of the blood resonance match the dynamics of the gas resonance and demonstrates efficient diffusion of xenon gas to the lung parenchyma and then to pulmonary blood. These observations demonstrate the utility of laser-polarized (129)Xe to detect exchange across the gas-blood barrier in the lungs and perfusion into myocardial tissue. Applications to measurement of lung function, kidney perfusion, myocardial perfusion, and regional cerebral blood flow are discussed. Magn Reson Med 42:1137-1145, 1999.

Relationship between reason for placement and medical findings among children in foster care.

OBJECTIVE: To determine the reasons for placement of children in foster care, the prevalence of medical findings during initial placement, and the relationship between reason for placement and medical findings. The association between placement reasons and parental substance abuse also was explored. METHODS: Population-based analysis of medical records of 749 children examined at the Child Protection Center in San Francisco from October 1, 1991, to December 31, 1992. Health evaluations consisted of a clearance examination of children during entry into foster care and a comprehensive examination 3 weeks later. Reasons for foster placement included abandonment, neglect, no available caretaker, physical abuse, sexual abuse, and failed placement. RESULTS: Nearly 50% of children in our study were < 6 years of age. Neglect (30%), physical abuse (25%), and no available caretaker (24%) were the most frequent placement reasons, followed by abandonment (9%), failed placement (7%), and sexual abuse (5%). Substance abuse was documented in 30% of parents, 51% when the placement reason was neglect. Medical findings were identified in 60% of children. Among 0 to 6-year-olds, 27% had upper respiratory illnesses, 23% had developmental delay, and 21% had skin conditions; for children 7 to 12 years of age, 32% failed vision screening, 12% had dental caries, and 11% had upper respiratory illnesses; and among 13- to 18-year-olds, 31% failed vision screening and 12% had positive tuberculin skin tests. For younger children, skin conditions were associated with neglect, no available care taker, and failed placement, and developmental delay with neglect and abandonment. For adolescents, history of psychiatric illness was associated with neglect and failed placement. Marks of abuse for all age groups were limited to children who had been physically abused. Three or more diagnoses were identified for approximately 20% of children who had been neglected or abandoned or had failed placement, compared with 10% of children who had been either physically or sexually abused. CONCLUSIONS: Specific medical findings associated with reasons for placement provide health professionals with additional information to assess more accurately the health care needs of children entering foster care. As important, screening tests revealed high rates of vision problems and exposures to tuberculosis, warranting earlier and more comprehensive screening. Finally, children who have endured variations of neglect or failed placement may have more health problems than anticipated previously.

Brain MRI with laser-polarized 129Xe.

The feasibility of brain MRI with laser-polarized 129Xe in a small animal model is demonstrated. Naturally abundant 129Xe is polarized and introduced into the lungs of Sprague-Dawley rats. Polarized xenon gas dissolves in the blood and is transported to the brain where it accumulates in brain tissue. Spectroscopic studies reveal a single, dominant, tissue-phase NMR resonance in the head at 194.5 ppm relative to the gas phase resonance. Images of 129Xe in the rat head were obtained with 98-microliter voxels by 2D chemical shift imaging and show that xenon is localized to the brain. This work establishes that nuclear polarization produced in the gas phases survives transport to the brain where it may be imaged. Increases in polarization and delivered volume of 129Xe will allow clinical measurements of regional cerebral blood flow.